Central Sierra Nevada

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1.5–3

  Old, weak snow lurks below several feet of storm snow in parts of the forecast area. These layers have gotten harder to break as the snow above them strengthens. Still, signs that these weak layers have reached their tipping point have occurred in some spots over the last few days, including:

  • Multiple snowmobile-triggered snowpack collapses (Blue Lakes 2/7)
  • A large snowmobile-triggered collapse/whumpf in which the snowpack dropped 1/4-1/2 an inch (Blue Lakes 2/6)
  • A large avalanche that might have failed on these layers during the storm (Mt. Lola overnight 2/5-2/6)
  • Long shooting cracks with remotely triggered cornice collapses (Tamarack Peak 2/5)
  • Numerous unstable snowpit test results

  This evidence suggests that persistent slab avalanches may be possible in some areas. NW-N-NE-E-facing slopes at elevations above about 8,000 ft. in the eastern and southern parts of the forecast area represent the places most likely to harbor this problem. Fewer signs of instability have been seen in the northern part of the forecast area. Below 8,000 ft, rain during the storms likely destroyed those layers or left a very strong, thick rain crust above them. 

  Uncertainty exists concerning this problem. Most people may not see any evidence of persistent slab instability today. It may take a larger trigger, like multiple people on a slope or another smaller avalanche, to cause one of these persistent slabs to fail. If they fail, these avalanches will be large and have serious consequences. These avalanches could be even larger in previously wind-loaded areas where dense wind slabs rest on top of the weak layers. The fragile cornices above those wind-loaded slopes could also provide the aforementioned large trigger. Some isolated unstable wind slabs may also linger in some complex or heavily wind-loaded terrain. 

  Unfortunately, a large avalanche on a seemingly stable slope might be the first and only clue that the slope is unstable. Due to this uncertainty and lack of feedback, dealing with persistent slabs is challenging. The most reliable way to reduce the potential consequences of an unexpected, persistent slab avalanche is to step back from the terrain where the problem might exist and avoid places with large potential triggers (like slopes under cornices).